Spatial coordinate measurement machines having a probe mounted at the end of an articulated, multi-sectional arm are extremely versatile and offer a great deal of flexibility for ascertaining the precise spatial coordinates of a plurality of locations dispersed about a workpiece or other body. The multiple articulations, typically four or more, of the arm allows the user to work around obstacles when positioning the probe on hard-to-reach locations. However, as the number of articulations and rotational axes are increased, the accuracy of the measurements are affected by the tolerance margin introduced by each articulation and its associated angular encoder. While the reaching ability of contemporary spatial measurement arms has been improved by increasing their freedom of movement about up to seven axes of rotation, a substantial loss of accuracy and reliability has been experienced.
When measuring locations around a workpiece that is relatively bulky compared to the size of the measurement arm, it becomes necessary to move the arm and its reference base to a different location so that all points to be measured can be conveniently reached. Each displacement of the measurement arm requires a readjustment of its reference point. The very measurement of the amount of displacement can introduce additional errors.
This invention results from an attempt to palliate loss of accuracy resulting from the multiplication of arm sections and rotational axes and to limit the size and number of required displacements of the measurement arm around a bulky workpiece.
The principal and secondary objects of this invention are to increase the range of a spatial measurement arm as well as limiting the degrees of motion necessary to reach all measurable locations on a workpiece while decreasing the required degree of travel of each arm section during a series of spatial coordinate measurements.
These and other valuable objects are achieved by a spatial coordinate measurement system which combines an articulated device mounting a measurement probe at the end of a multi-sectional arm, an electronic data processor and a rotatable workpiece carrier whose angular position around an adjustably positionable axis is indicated by an angular encoder. The electronic data processor uses the output of the angular encoder to translate, not physically but virtually, the origin or reference point of the spatial measurement device. Accordingly, the carrier and the workpiece can be conveniently rotated to place any situs to be measured within a short reach from the measurement device probe.
The position of the carrier and of its axis of rotation can be indiscriminatly set allowing for maximum flexibility of movement of the workpiece. These positions are entered into the system by first measuring the spatial location of a reference locus, preferably marked on the carrier, at three or more different angular orientations. A program in the electronic data processor uses the results of those three or more measurements to establish the exact orientation and position of the axis which are taken into account during all successive measurements regardless of the rotational orientation of the carrier. That rotational orientation is monitored by another program routine in the electronic data processor and used to virtually translate the coordinate location of the measurement device origin. As the carrier and workpiece are rotated, the measurement device and its origin appears to rotate around the same axis and in the same degree of circular travel. The carrier and workpiece rotational information is also fed to the Computer Assisted Drawing (CAD) system which is part of the electronic data processor so that the monitor displays a corresponding rotational movement of the workpiece representation.